One strategy for treating human diseases is to target specific disease-associated genes by either replacing impaired gene functions or by suppressing unwanted gene functions. Expression vectors are commonly used for introducing active genes into a cell to provide missing gene functions. To suppress unwanted gene functions, antisense oligonucleotides, antibodies, and small molecule drugs are often used as therapeutic agents.
Applications of RNA interference (RNAi) (Elbashir et al. (2001) Nature 411: 494-498) and deoxyribonucleotidylated-RNA interfering (D-RNAi) (Lin et al. (2001) Biochem. Biophys. Res. Commun. 281: 639-644) technologies in treating human diseases are also in progress. RNAi elicits post-transcriptional gene silencing (PTGS) phenomena, knocking down specific gene expression with high potency and less toxicity than traditional antisense gene therapies. However, the gene silencing effects mediated by dsRNA are repressed by interferon-induced global RNA degradation when the dsRNA size is larger than 25 base pairs (bp), especially in mammalian cells. Although transfection of short interfering RNA (siRNA) or microRNA (miRNA) of less than 21 bp can overcome interferon-associated problems, the size limitation impairs the usefulness of RNAi, as it is difficult to deliver such small and unstable dsRNAs in vivo due to high dsRNase activities in human bodies (Brantl (2002) Biochimica et Biophysica Acta 1575: 15-25). Therefore, there remains a need for a more effective and reliable gene modulation system.